In the prior art, only UC2 was measured, and, in relation to this, the time dte for which the switch remains closed was regulated. When designing the circuit, however, a worst-case design must be carried out which takes into account different causes of fluctuations in UC1. Firstly, UC1 may change when the rated voltage changes when crossing over national boundaries, and may also have tolerances of up to plus/minus 15 percent at a given rated voltage. If, for example, the rated voltage UC1 is assumed to be 240 V, this voltage at the system peak may be 276 V multiplied by the square root of 2, that is approximately 390 V. A design for UC2 for this operation would therefore be approximately 425 V. At a different rated voltage of 230 V and without any tolerances, UC1 at the system peak would be equal to 230 V multiplied by the square root of 2=325 V and an optimum design for UC2 for this rated operation would therefore be approximately 355 V. As already mentioned, however, in a design without regulation the worst case would have to be assumed here for UC1. Since, furthermore, the tolerance of the measurement of UC2 and the ripple voltage of UC2 need to be taken into account, as a result in normal operation over 70 V are given away, as it were.
Another solution is to measure UC1 and UC2 and to regulate the time dte for which the switch S remains closed such that the difference between the two voltages is kept to a minimum. Since, however, the two voltages are measured at the reference potential, where the difference needs to be very small, the tolerances of the voltage measurements need to be taken into account. In the event of small differences, this results in a multiplication of the tolerance of the difference. This solution therefore does not provide an optimum circuit design, either, and therefore still results in undesirably high power loss and its inherent consequences.
In relation to the prior art, reference is also made to WO 01/69984 A2.